Gene expression in diseased and healthy cells and in cells in different stages of development is often different. The ability to monitor gene expression in such cases provides researchers and medical professionals with a powerful diagnostic tool. One can monitor gene expression, for example, by measuring the presence or absence of a nucleic acid (e.g., a mRNA) that is the transcription product of a gene of interest. Monitoring the nucleic acid may be accomplished by chemically or biochemically labelling the mRNA with a detectable moiety followed by hybridization to a nucleic acid probe for the gene. The detection of a labelled nucleic acid at the probe position indicates that the targeted gene has been expressed.
Various methods of RNA detection have been developed. These include the “Northern” blotting procedure and the use of radioactive isotopes such as 32P. Non-radioactive detection techniques have also been developed. Langer et al., Proc. Natl. Acad. Sci. USA 1981, 78, 6633-6637, for example, disclosed certain biotin labelled nucleosides. Lockhart et al., U.S. Pat. No. 6,344,316, disclosed enzymatic methods of end-labelling with non-radioactive nucleotides. Igloi et al, Anal. Biochemistry, 233, 124-9, 1996, disclosed methods for non-radioactive labelling of RNA. Wang et al, RNA (2007), 13, 1-9, disclosed methods for a microRNA profiling assay.
There remains, however, a need for nucleic acid, e.g., RNA, labelling reagents that can be used for efficient and accurate labelling and monitoring of gene expression.